1. Field of the Invention
The present invention relates generally to capacitors which store electrical energy by electrochemical means as opposed to conventional capacitors which store energy electrostatically. More particularly, the present invention is directed to a new type of improved electrochemical capacitor which is capable of being charged and discharged at relatively rapid rates.
2. Description of the Background Art
Capacitors based on electrochemical (as opposed to electrostatic) energy storage promise to solve two major problems facing the development of space power systems. First, conventional capacitor materials will not withstand the 300.degree. C. ambient temperatures expected on space platforms. Second, weight and volume constraints in space will demand capacitors with the maximum possible energy density.
The ability of electrochemical devices to perform at elevated temperatures has already been demonstrated. Examples include the sodium-sulfur battery (300.degree.-400.degree. C.), the molten-carbonate fuel cell (700.degree.-800.degree. C.) and the solid-oxide fuel cell (900.degree.-1000.degree. C.). Similarly, the energy density of electrochemical storage cells can exceed 1000 kJ/kg--several orders of magnitude greater than that of conventional capacitors. The one factor that has prevented widespread use of electrochemical cells in place of capacitors is their relatively low power density (long charge/discharge times).
Two electrochemical approaches have been taken towards combining the high energy density of batteries with the high power density of capacitors. The first approach has involved maximizing the energy density of electrostatic storage (the "double-layer capacitor" approach).
The very large capacitance values associated with electrochemical "double layers" (typically 10-20 uF cm.sup.-2) arise from the small effective thickness of the "dielectric"-a monolayer of water molecules absorbed on the electrode surface. The phenomenon has been successfully incorporated in a class of energy storage devices known as double layer capacitors. An example of this type of capacitor is the Maxcap.TM. capacitor marketed by SOHIO Engineered Materials Co. (See SOHIO Engineered Materials Co. Brochure, "Maxcap.TM. Double Layer Capacitor", Form A-14,036, January, 1985.) (Also see A. Nishino, A. Yoshida, I. Tanahashi, I. Tajima, M. Yamashita, T. Muranada, and H. Yoneda, National Technical Report, 31, 318 (1985)).
In the Maxcap.TM. device, pairs of carbon black electrodes, separated by an electrolyte, are charged to a point just short of water electrolysis. With electrode surface areas in excess of 1000 square meters per gram (m.sup.2 g.sup.-1), the device offers energy densities in the range of 1 kilojoule per kilogram (kJ kg.sup.-1), or about 0.3 watt-hours per kilogram (W h kg.sup.-1). This range approaches the energy densities of conventional batteries and is half an order of magnitude greater than achievable with conventional capacitors. However, charge and discharge times are relatively long, being on the order of 10 seconds. The slow charging and discharging rates results from the "transmission line" effect of the distributed capacitances and large series resistances that are intrinsic to porous electrode systems.
A second approach relies on the electrochemical phenomenon known as "pseudocapacitance". Here, the reversible relationship between charge and voltage arises from the kinetically rapid oxidation and reduction of species that are chemically bound to the electrode surface. Capacitors based on the pseudocapacitance phenomenon are disclosed in European Patent Application No. 82109061.0 (Publication No. 0078404) which was filed on Sept. 30, 1982.
The pseudocapacitance type capacitors disclosed in the European Patent Application No. 82109061.0 are designed to provide a high degree of kinetic and/or coulombic reversibility. The disclosed capacitors include electrodes made of materials such as ruthenium oxides and mixtures of ruthenium with tantalum and/or iridium which are spaced 0.010 inch (10 mil.) and 0.015 inch (15 mil.) apart. A wide variety of electrolytes are suggested for use. Although these pseudocapacitance type capacitors may be well suited for their intended purposes, they are not suitable for use in systems, such as rail guns, which require rapid pulse power at intervals ranging from a few milliseconds to a few seconds.
Accordingly, there is presently a need for electrochemical capacitors which are compact, lightweight and capable of being charged and discharged during periods ranging from a few milliseconds to a few seconds.